Wearable potentiometric ion sensors

Abstract

Wearable potentiometric ion sensors (WPISs) have become an exciting analytical platform that combines chemical, material and electronic efforts to supply physiological information during certain human activities. The real possibility of wearing an analytical device with diverse configurations—sweatband, patches, garments—without disturbing the welfare of the carrier has enabled potentiometric ion sensors both as health quality and sport performance controllers. Recent studies show a large involvement of WPISs in the following of critical biomarkers (timely or continuously), such as sodium, potassium, calcium, magnesium, ammonium and chloride, which are present at relatively high concentrations in sweat (∼mM levels). Certainly, the non-invasive nature of WPISs and other significant features, e.g., simplicity and cost-effectiveness, have broadened new horizons in relation to applied analytical chemistry. This has been pointed out in the literature over the last decade with the predominance of two analytical outcomes: (i) the improvement of sport performance as a result of continuous detection of ions in sweat (health status of the individual) while decreasing physiological complications (injuries, muscle cramps, fatigue and dehydration) during practice; and (ii) advancements in clinical diagnostics and preventive medicine as a consequence of the monitoring of the health status of patients suffering from any kind of disorder. Beyond the undeniable importance of the integration of WPISs to satisfy current societal needs, the following crucial questions about misleading and missing analytical features need to be answered: To what extent is WPIS technology a reliable analytical tool for the quantification of ions? Is cross-validation the current bottleneck toward further progress? Which are the fundamental steps involving the ion-selective electrode side that would benefit WPIS outcomes? Why is sweat the main (and almost the only) biological fluid to be monitored by WPISs? What is the best sampling strategy to be incorporated into WPIS devices for on-body monitoring of sweat? Which precision limits should be considered to assure a reliable decision-making process? Accordingly, this review focuses on the progression of WPISs from an analytical perspective—merely our vision of the field—within the period between 2010 and 2018. An updated search using specific keywords (wearable, ion, potentiometry, sensor) provided 43 contributions, which are herein highlighted, with a sustainable acceleration over the last three years. Thus, this review describes the current state of WPIS technology, the construction of wearable all-solid-state potentiometric sensors, critical requirements of potentiometric sensors to be fulfilled in a wearable configuration and key features regarding the ideal implementation of WPISs as reliable messengers of physiological information in real scenarios.